Radar level gauge systems are today in use in a variety of fields of application for accurate level determination, as well as for determination of other product parameters, such as temperature, flow etc. For determining the level of a product by means of a radar level gauge system, electromagnetic signals are transmitted and propagated, usually by means of an antenna, towards a surface of the product, where signals are reflected. The reflected signals are received by the radar level gauge system, and the distance between a reference position and the surface of the product is determined by comparing the transmitted signals with the reflected signals. Based on this distance, the filling level can be determined.
Especially in open or semi-open applications, such as open tanks, floating-roof tanks, reservoirs, or even rivers or lakes, the operation of a radar level gauge system is typically subject to government regulations in respect of such parameters as the frequency and power of the transmitted signals.
According to such existing and/or anticipated regulations, the transmission power should be limited to a certain level. At the same time, the quality of measurement, such as the accuracy with respect to distance, of the radar level gauge system should not be sacrificed to obtain this transmission power level.
Such a limited transmission power in combination with high quality measurement is readily achievable in a radar level gauge system having a transceiver with separated transmitter output and receiver input, the output being connected to a transmission antenna, and the input to a receiver antenna, but considerably more difficult to achieve for a transceiver having a common transceiver input/output terminal.
In present solutions for achieving an low transmission power level in a radar level gauge having such a transceiver with a common input/output terminal, and a single antenna, it has proven to be very difficult, if at all possible, to at the same time achieve a sufficiently low output power, say −40 dBm (100 nW) or lower and a sufficiently accurate measurement of the filling level.
One such existing solution is schematically illustrated in FIG. 1. In FIG. 1, a transceiver 10 is shown, having a signal generator 11 connected to power dividing circuitry, here in the form of a Wilkinson Power Divider (WPD) 12. After the WPD 12, the line is divided into a transmitter branch 13 and a receiver branch 14. The transmitter branch 13 and the receiver branch 14 are connected to a transceiver input/output terminal 16 via a second WPD 19. In order to attenuate the transmitted electromagnetic signals, resistive damper circuits, or so-called pads, 15a-d are provided on the transmitter branch 13. Typically, using surface mounted components as damper circuits 15a-d, an attenuation of about 20 dB can be achieved.
After having been attenuated in the transmitter branch 13, the transmitted electromagnetic signals pass the second WPD 19 on their way towards the antenna 3.
As for reflected electromagnetic signals picked up by the antenna 3, these signals are divided by the second WPD 19, and the fraction of the reflected signals going into the receiver branch are amplified by an amplifier 17 and mixed with signals from the signal generator 11 in a mixer 18. The distance to the relevant surface can then be determined based on the output from the mixer 18.
In this solution, however, the desired combination of low transmission power and high quality of measurement is difficult to obtain.